The handling and disposal of bio-solids, such as human waste, presents a major problem. One technique utilized is to form the bio-solids into a wet sludge having approximately an 18% to 35% concentration of the bio-solids and to process the sludge into dry pellets. The pellets contain nitrogen in slow release form. They can be sold to fertilizer manufacturers for mixing with standard fertilizers or for use directly as a general fertilizer applied to agricultural areas.
In a standard plant for producing the pellets, the finished pellets must have a certain specific density (solid content as compared to liquid) and size. If the final product in the pelletizing plant does not meet the density and size specifications, then the pellets must be reprocessed. This is usually accomplished by returning the reject pellets to mix with incoming wet sludge to be recoated with additional sludge and recooked in a dryer to correct the proportion of density and size.
Many times, the reject pellets to be reprocessed amount to 60%-65% of the output of the pellet forming operation. The pellets to be reprocessed, which are dry, are transported vertically to a raised storage bin. This is usually accomplished by a bucket elevator. Such transport produces a substantial amount of dust and spill of the pellets to be reprocessed. From the bin, the pellets are distributed by gravity feed to a mixer, typically of the paddle or pin type. The wet sludge is also transported to the mixer. The mixer must insure that the dry pellets being reprocessed are evenly coated by the wet sludge and have achieved a size of to 3 to 4 mm. From the mixer the reprocessed material, now containing the added wet sludge, is transported to a high heat dryer.
This process of the prior art has a drawback in that the dust and spills produced possibly can give rise to a flashback explosion. Also, the process is lacking in providing different ways of adjusting the moisture and recoated sludge content of the reprocessed pellets.